postprocessing the solr documents are now not completely retrieved.
instead, only fiels, needed for the postprocessing are extracted. When
Solr document are written, this is done using partial updates.
This increases postprocessing speed by about 50% for embedded Solr
configurations. For external Solr configurations the enhancement should
be much higher because the postprocessing with remote Solr is very slow.
When doing partial updates to a remote Solr, this method should perform
much better than before, it is expected that this is even much higher
than the increase with local Solr.